The present invention relates to a frequency-stabilized laser source using a solid state laser adapted to be used as a frequency standard, in particular in the field of telecommunications.
The very rapid expansion of local telecommunications networks of the local area network (LAN) type, and of greater-distance telecommunications networks of the wide area network (WAN) type is leading to an ever-increasing requirement for bandwidth. In order to satisfy this requirement for bandwidth in telecommunications networks, one technique that is commonly implemented at present lies in developing networks that are wavelength division multiplexed (WDM). With such wavelength division multiplexing, a plurality of signals (or channels) are transported simultaneously in a single optical fiber, but at different wavelengths.
Multichannel light transmission (WDM) raises two major difficulties. The first lies in creating and extracting each channel at reasonable cost without losing information. The second difficulty is associated with optically amplifying all of the channels without distortion and/or loss of information. These difficulties increase with an increase in the number of channels used.
A first technique commonly implemented for selecting channels (multiplexing or demultiplexing) is a passive frequency-filtering method based on using Bragg gratings. At present, although Bragg grating technology is quite well mastered, and of reasonable cost, certain physical limitations (long-term stability in Bragg wavelength, sensitivity to external media, in particular temperature, . . . ) are leading to several problems, particularly when channel density becomes high.
A second technique that can be used for avoiding problems associated with using Bragg gratings is a method in which information is processed actively, based on heterodyne detection. However, implementing that method to discriminate precisely between channels requires frequency standards to be available in the form of longitudinal monomode laser sources having very high spectral purity, small size, and suitable for being integrated in telecommunications racks, which sources must be accurately stabilized in frequency, must emit at frequencies that are known with precision, and must be located on the various different sites of the telecommunications network (both for transmission and for reception). In practice, given the wavelengths presently used in the field of telecommunications, these standard laser sources must enable a coherent light beam to be emitted at a wavelength of 1.56 micrometers (μm). The greater the precision of the standard source, the closer the wavelengths of the various channels can be to one another, thus making it possible to increase overall bandwidth significantly.
At present, in the field of telecommunications by optical fibers, the laser sources most commonly used use a laser of the semiconductor type as means for emitting the light beam, and more particularly they use a laser diode. A very wide range of laser diodes can be found on the market. The advantages of such diodes are their simplicity of use and their compactness. In contrast, a drawback of such laser diodes is the poor definition of their emission wavelength (wavelength fluctuates over time under the influence of external parameters such as temperature, mechanical deformation, aging, . . . ).
Proposals have also recently been made for a laser source to be provided using a solid state erbium-ytterbium laser stabilized in frequency at about 1.5 μm. That laser source is described in the article entitled:
“Frequency stabilization of a novel 1.5 Er-Yb bulk laser to a 39K sub-Doppler line at 770.1 nm” by Sveltco et al., published in IEEE Journal of Quantum Electronics, April 2001, IEEE, USA, Vol. 37, No. 4, pp. 505–510.
Frequency stabilization of the laser source as described in that article is implemented by locking the frequency of the laser beam on a transition T of an absorber chemical element (specifically rubidium). More precisely, it is recommended to perform such lock by implementing synchronous detection based on frequency modulation of the laser beam delivered by the solid state laser (see paragraph V.: “Wavelength-modulation spectroscopy and frequency-locking experiment”). As a result, the frequency of the laser beam delivered by the solid state laser oscillates continuously and is therefore not accurately stationary. Although the laser source described is indeed frequency stabilized, it is nevertheless not adapted to constituting a frequency standard because of the continuous fluctuation in its frequency.
In the field of solid state lasers, the main object of the present invention is to propose a novel frequency-stabilized laser source that is adapted for use as a frequency standard.
Another object of the invention is to propose a novel frequency-stabilized laser source that is adapted for use as a frequency standard in the field of telecommunications.
Another object of the invention is to propose a novel frequency-stabilized laser source that enables transverse monomode radiation to be emitted so as to facilitate coupling the electromagnetic waves with optical fibers and planar waveguides.
Another object of the invention is to propose a novel frequency-stabilized laser source that enables longitudinal monomode radiation to be emitted and good spectral fineness to be obtained.
The solution of the invention consists in providing a laser source that combines the technical characteristics of claim 1. The solution of the invention is thus based on locking the frequency of the laser beam delivered by a solid state laser on a transition of an absorber chemical element, which lock advantageously combines both operation with saturated absorption (said transition being saturated by the beam passing through the absorber chemical element) and synchronous detection based on modulating said saturated transition. This produces a single frequency laser beam whose output frequency is stationary compared with that of the above-mentioned laser source of Sveltco et al., i.e. its frequency is not continuously modulated, and its frequency is much more precise than that which would be obtained operating with single-pass absorption only. The laser source is thus perfectly adapted for making a frequency standard.